Geographic Range

The Wabash pigtoe is found in the Mississippi drainage from Oklahoma and Tennessee to Minnesota and Pennsylvania. In the St. Lawrence it is found from Wisconsin to central New York and southern Ontario. In Canada it is found in the the Red River of the North, the Nelson River and Lake Winnipeg.

In Michigan, F. flava is found in the lower peninsula, from Muskegon River tributaries through the southern portion of the state. In the upper peninsula, it has been recorded in the Menominee and Sturgeon Rivers and the mouth of the Whitefish River. (Burch, 1975)

Habitat

The Wabash pigtoe is found from creeks to large rivers with moderate current in mud, sand or gravel. However, it is more common in streams than rivers. On the Huron River, it seemed to prefer a firm bottom of sand and fine gravel. In the Clinton it was found on a firm sand bottom. (Cummings and Mayer, 1992; van der Schalie, 1938; Watters, 1995)

Physical Description

The Wabash pigtoe is up to 7.6 cm (3 inches) long , and is triangular to quadrate in shape. The shapes of headwater to river forms vary. Headwater forms are more rounded and river forms have a more prominent posterior ridge. The shell is fairly thick and compressed. The
anterior end is gently rounded, the posterior end bluntly pointed. The dorsal margin is short and straight. The ventral margin is rounded anteriorly and bluntly pointed posteriorly.

Umbos are broad and raised slightly above the hinge line. The beak sculpture has three to four weak concentric bars.

On the inner shell, the
left valve has two
pseudocardinal teeth, which are heavy, rough, and serrated. The anterior tooth is narrow and elongate, the posterior one is heavy and triangular. The two lateral teeth are short and straight. The right valve has one large, serrated, pseudocardinal tooth, and one (sometimes two) are short, straight and stout.

The beak cavity is deep and wide. Although the nacre is white, occasionally it is has a pink or salmon tint and is iridescent posteriorly.

In Michigan, identification of F. flava can be easily confused with Pleurobema sintoxia, particularly in upper reaches of river systems. The sulcus of F. flava becomes more prominent in more downstream populations, making it more distinguishable from P. sintoxia. The Wabash pigtoe generally has a deeper beak cavity. When present in females, glochidial packets from the Wabash pigtoe also tend to be orangish while the round pigtoe has white glochidial packets. (Cummings and Mayer, 1992; Oesch, 1984; Watters, 1995)

Development

Fertilized eggs are brooded in the marsupia (water tubes) up to three months, where they develop into larvae, called glochidia. The glochidia are then released into the water where they must attach to the gill filaments and/or general body surface of the host fish. After attachment, epithelial tissue from the host fish grows over and encapsulates a glochidium, usually within a few hours. The glochidia then metamorphoses into a juvenile mussel within a few days or weeks. After metamorphosis, the juvenile is sloughed off as a free-living organism. Juveniles are found in the substrate where they develop into adults. (Arey, 1921; Lefevre and Curtis, 1910)

Reproduction

Age to sexual maturity for this species is unknown. Unionids are gonochoristic (sexes are separate) and viviparous. The glochidia, which are the larval stage of the mussels, are released live from the female after they are fully developed.

In general, gametogenesis in unionids is initiated by increasing water temperatures. The general
life cycle of a unionid, includes open fertilization. Males release sperm into the water, which is taken in by the females through their respiratory current. The eggs are internally fertilized in the suprabranchial chambers, then pass into water tubes of the gills, where they develop into glochidia.

Females brood fertilized eggs in their marsupial pouch. The fertilized eggs develop into glochidia. There is no parental investment after the female releases the glochidia.

Parental Investment

pre-fertilization

provisioning

pre-hatching/birth

provisioning

female

Lifespan/Longevity

The age of mussels can be determined by looking at annual rings on the shell. However, no demographic data on this species has been recorded.

Behavior

Mussels in general are rather sedentary, although they may move in response to changing water levels and conditions. Although not thoroughly documented, the mussels may vertically migrate to release glochidia and spawn.

Fusconaia flava releases small, white, worm-like glochidial packets which resemble invertebrates that fish would eat. These mimics are likely used to get the glochidia to the host fish. (Oesch, 1984)

Communication and Perception

The middle lobe of the mantle edge has most of a bivalve's sensory organs. Paired
statocysts, which are fluid filled chambers with a solid granule or pellet (a statolity) are in the mussel's foot. The statocysts help the mussel with georeception, or orientation.

Mussels are heterothermic, and therefore are sensitive and responsive to temperature.

Unionids in general may have some form of chemical reception to recognize fish hosts. How the Wabash pigtoe attracts or recognizes its fish host is unknown.

Food Habits

In general, unionids are filter feeders. The mussels use cilia to pump water into the
incurrent siphon where food is caught in a mucus lining in the demibranchs. Particles are sorted by the
labial palps and then directed to the mouth. Mussels have been cultured on algae, but they may also ingest bacteria, protozoans and other organic particles.

The parasitic glochidial stage absorbs blood and nutrients from hosts after attachment. Mantle cells within the glochidia feed off of the host’s tissue through phagocytocis. (van der Schalie, 1938; Watters, 1995)

Ecosystem Roles

Fish hosts are determined by looking at both lab transformations and natural infestations. Looking at both is necessary, as lab transformations from glochidia to juvenile may occur, but the mussel may not actually infect a particular species in a natural situation. Natural infestations may also be found, but glochidia will attach to almost any fish, including those that are not suitable hosts. Lab transformations involve isolating one particular fish species and introducing glochidia either into the fish tank or directly inoculating the fish gills with glochidia. Tanks are monitored and if juveniles are later found the fish species is considered a suitable host.

Conservation Status

Contributors

Renee Sherman Mulcrone (author).

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

detritivore

an animal that mainly eats decomposed plants and/or animals

detritus

particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

filter-feeding

a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.

freshwater

mainly lives in water that is not salty.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

Disclaimer:
The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control.

This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.